Display device

ABSTRACT

The invention provides a method and apparatus to obtain accurate gradation by obtaining an accurate ratio of light emitting parts in a display device which implements gradation by forming a plurality of TFTs and a plurality of OELDs in each pixel, directly connecting the TFTs and OELDs, and switching an on and off state of the TFTs, and controlling a light emitting area of the OELDs. A plurality of OELDs have the same shape, and gradation can be implemented by controlling the number of OELDs that emit light. A plurality of OELDs have a round shape. A plurality of OELDs are arranged at the same interval in a vertical and/or horizontal direction. According to this structure, because the light emitting areas of the plurality of OELDs become equal to each other, by controlling the number of OELDs, a ratio of the light emitting areas can be accurately obtained.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] This invention relates to a display device, particularly a thinfilm transistor driven organic electro-luminescent display device(hereafter referred to as TFT-OELD) which is driven by a thin filmtransistor (hereafter referred to as TFT) and provided with an organicelectro-luminescent element (hereafter referred to as OELD) of a highpolymer system formed in a liquid phase process.

[0003] 2. Description of Related Art

[0004] TFT-OELD is promising because it is a display device whichrealizes light-weightness, thinness, smallness, higher accuracy, widerview angle, lower electric consumption, and the like. FIG. 1 shows aconventional TFT-OELD. FIG. 2 shows a cross-sectional view of theconventional TFT-OELD. Here, there is only one pixel 11 depicted, butthere are actually many pixels 11 in plural rows and lines. Here, OELD18 is a high polymer system, formed by a liquid phase process, such asspin coating, blade coating, ink jet, or the like.

[0005] In order to implement gradation, in the case of the structureshown in FIG. 1, a gate voltage of a driving TFT 17 is made to changeand conductance is changed, so electric current which flows in the OELD18 needs to be controlled. However, gradation according to this method,particularly in half tone, result in irregularity of transistorcharacteristics of the driving TFT 17 and appears as brightnessirregularity of the OELD 18, and there is a problem such that the screenbecomes non-uniform.

[0006] Therefore, as shown in FIG. 3, a method is considered whichimplements gradation by changing a light emitting area of the OELD 18(Japanese Patent Application 9-233107). FIG. 4 shows a driving method ofthis method. A scanning electric potential 31 is applied to a scanningline 12, and a signal line 13 is formed of a signal line (lower bit) 131and a signal line (upper bit) 132. A signal electric potential (lowerbit) 321 and signal electric potential (upper bit) 322 are respectivelyapplied as a signal electric potential 32. A driving TFT 17 is formed ofa driving TFT (lower bit) 171 and a driving TFT (upper bit) 172, and theOELD 18 is formed of an OELD (lower bit) 181 and an OELD (upper bit)182. In this example, 2-bit 4 gradation is considered, so an area ratiobetween OELD (lower bit) 181 and OELD (upper bit) 182 is 1:2.

[0007] In this method, the driving TFT 17 takes either a substantiallycompletely on state or a substantially completely off state. In the onstate, the resistance of the driving TFT 17 is small enough to beignored, compared to the resistance of OELD 18, and the electric currentamount which flows in the driving TFT 17 and OELD 18 is substantiallydetermined by only the resistance of the OELD 17.

[0008] Therefore, irregularity of transistor characteristics of thedriving TFT 17 does not appear as brightness irregularity of the OELD18. Furthermore, in the off state, voltage applied to the OELD 18becomes less than a threshold voltage, so the OELD 18 does not emitlight at all, and, needless to say, irregularity of transistorcharacteristics of the driving TFT 17 does not appear as brightnessirregularity of the OELD 18.

[0009]FIG. 5 is a cross-sectional view of TFT-OELD which implementsgradation display by changing a light emitting area of the OELD 18 shownin FIGS. 3 and 4. FIG. 5(a) is a cross-sectional view of the OELD (lowerbit) 181, and FIG. 5(b) is a cross-sectional view of the OELD (upperbit) 182. The ratio between the light emitting part 25 of the OELD(lower bit) 181 and the light emitting part 25 of the OELD (upper bit)182 is preferably 1:2.

[0010] A light emitting layer 22 is an OELD of a high polymer system andformed in a liquid phase process. A surface of a bank 24 is lyophobicand the light emitting layer 22 does not remain. Therefore, the area ofthe OELD 18 is determined by patterning. With respect to a side surfaceof the bank 24, the materials and processing determine whether the sidesurface of the bank 24 becomes lyophobic or lyophilic.

[0011]FIG. 5 shows the case of a lyophilic side surface of the bank 24.As a phenomenon that is characteristic of a liquid phase process, thelight emitting layer 22 has a cross-sectional shape which is pulledtoward the side surface of the bank 24. In this case, electric currentflows into a thinner part of the light emitting layer 22, and this partbecomes a light emitting part 25. The cross-sectional shape of the lightemitting layer 22 described here is sensitive to liquid amount, liquidmaterial, an initial position of the liquid, and a state, temperature,atmosphere, or the like of a substrate, and which are difficult tocontrol. That is, it is difficult to obtain an absolute value of adesired light emitting area. Because of this, it is difficult to obtainan accurate ratio of 1:2, between the light emitting part 25 of the OELD(lower bit) 181 and the light emitting part 25 of the OELD (upper bit)182, and ultimately, it is difficult to obtain accurate gradation.

[0012]FIG. 6 is a cross-sectional view of OELD (lower bit) 181 (FIG.6(a)) and a cross-sectional view of OELD (upper bit) 182 (FIG. 6(b)) inthe same manner as in FIG. 5. In FIG. 6, the side surface of the bank 24is lyophobic. As a phenomenon that is characteristic of a liquid phaseprocess, the light emitting layer 22 has a cross-sectional shape whichis distant from the side surface of the bank 24. In this case as well,electric current flows into the thinner part of the light emitting layer22, and this part becomes the light emitting part 25. In this case aswell, in the same manner as in the case of FIG. 5, it is difficult toobtain an accurate ratio of 1:2 between the light emitting part 25 ofthe OELD (lower bit) 181 and the light emitting part 25 of the OELD(upper bit) 182, so it is difficult to obtain accurate gradation.

SUMMARY OF THE INVENTION

[0013] Therefore, one aspect of this invention is to obtain an accurateratio of the light emitting parts, and accurate gradation. Therefore,the invention may provide a display device in which gradation isimplemented by forming a plurality of TFTs and a plurality of OELDs ineach pixel, directly connecting the TFTs and OELDs, switching an on andoff state of the TFTs, and controlling an area of the OELDs, that emitslight, wherein the plurality of OELDs have the same shape, and gradationis implemented by controlling the number of OELDs that are created toemit light and by controlling an appropriate on/off state of the TFTconnected to each OELD.

[0014] According to this structure, as a characteristic phenomenon of aliquid phase process, even if an OELD becomes a cross-sectional shapewhich is pulled in to a side surface of a bank or is distant from theside surface of the bank, the light emitting part of each OELD is thesame area, and accurate gradation can be obtained. In this structure aswell, it is difficult to obtain an absolute value of a desired lightemitting area, but the light emitting area of a plurality of OELDsbecomes equal, so the ratio of the light emitting areas can be accurateby controlling the number OELDs.

[0015] The display device may also include a plurality of OELDs thathave a round shape. According to this structure, the light emitting partof each OELD can reliably be the same area, and accurate gradation canbe obtained. The reasons are as follows. When an OELD has a shape with arectangular vertex (or vertices), there is a possibility that aphenomenon may occur, for example, that the vertex becomes pulled in orthe vertex cannot be filled. This phenomenon prevents a user fromobtaining accurate gradation for the same reason as in the problems of across-sectional shape as described above. This phenomenon is moresensitive to the liquid amount, liquid material, initial position ofliquid, and the state, temperature, and atmosphere of a substrate, moreso than the problems in a cross-sectional shape described above, and itis difficult to control this phenomenon between adjacent OELDs. Bymaking the OELD round shaped, this phenomenon can be avoided.

[0016] The display device may also include a plurality of OELDs arearranged at the same interval in a horizontal and/or vertical direction.According to this structure, the light emitting part of each OELD ismade to be more reliably the same area, and accurate gradation can beobtained. The reasons are as follows. When OELD is formed by spincoating or blade coating, the light emitting layer which is coated overall the pixels, due to the lyophobicity of a surface of the bank, thelight emitting layer naturally flows into a convex part of the bank. Inthe case of inkjet as well, this may sometimes happen. At this time,when a concave area surrounded by a bank convex part is large, the lightemitting layer coated over this part flows into a convex bank portion,so the light emitting layer becomes thick. When the convex areasurrounded by the bank concave part is small, the light emitting layerbecomes thin. Ultimately, irregularity of film thickness of the lightemitting layer is generated. This irregularity can be avoided byarranging a plurality of OELDs at the same interval in a horizontal orvertical direction.

[0017] Additionally, according to this structure, when the OELDs areformed by an ink jet process, ink jetting can be performed at the sameinterval, so fabrication can be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a diagram showing a pixel of conventional TFT-OELD whichimplements a gradation display by changing a TFT conductance;

[0019]FIG. 2 is a cross-sectional view of a conventional TFT-OELD;

[0020]FIG. 3 is a diagram showing one pixel of a TFT-OELD whichimplements a gradation display by changing a conventional OELD lightemitting area;

[0021]FIG. 4 is a diagram showing a TFT-OELD driving method whichimplements a gradation display by changing a conventional OELD lightemitting area;

[0022] FIGS. 5(a) and 5(b) are cross-sectional views of an OELD in aTFT-OELD which implements a gradation display by changing a conventionalOELD light emitting area (when the side surface of the bank islyophilic), where FIG. 5(a) is a diagram of an OELD (lower bit) and FIG.5(b) is a diagram of an OELD (upper bit);

[0023] FIGS. 6(a) and 6(b) are cross-sectional views of an OELD in aTFT-OELD which implements a gradation display by changing a conventionalOELD light emitting area (when the side surface of the bank islyophobic), where FIG. 6(a) is a diagram of an OELD (lower bit), andFIG. 6(b) is a diagram of an OELD (upper bit);

[0024]FIG. 7 is a diagram showing one pixel of a TFT-OELD related toembodiment 1 of this invention;

[0025]FIG. 8 is a diagram showing one pixel of a TFT-OELD related toembodiment 2 of this invention; and

[0026]FIG. 9 is a diagram showing one pixel of a TFT-OELD related toembodiment 3 of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0027] The following explains preferred embodiments of this invention,based upon the drawings.

[0028]FIG. 7 is a diagram showing a pixel of a TFT-OELD related to afirst embodiment of this invention. Here, only one pixel 11 is depicted,but many pixels 11 actually exist in a plurality of rows and a pluralityof lines.

[0029] An OELD (lower bit) 181 is formed of OELD (lower bit-rectangular)1811, and an OELD (upper bit) 182 is formed of OELD (upperbit.first.rectangular) 18211 and an OELD (upper bit.second.rectangular)18221. The OELD (lower bit.rectangular) 1811, the OELD (upperbit.first.rectangular) 18211, and the OELD (upperbit.second.rectangular) 18221 have the same shape, so the same lightemitting area can be obtained, and accurate gradation can be obtained bychanging the number of OELDs that are caused to emit light.

[0030]FIG. 8 is a diagram showing a pixel of a TFT-OELD related toanother embodiment of this invention. Here, only one pixel 11 isdepicted, but many pixels 11 actually exist in a plurality of lines anda plurality of rows.

[0031] An OELD (lower bit) 181 is formed of an OELD (lower bit.roundshape) 1812, and an OELD (upper bit) 182 is formed of OELD (upperbit.first.round shape) 18212 and OELD (upper bit.second.round shape)18222. The OELD (lower bit.round shape) 1812, the OELD (upperbit.first.round shape) 18212, and the OELD (upper bit.second.roundshape) 18222 have the same round shape, so the same light emitting areacan be reliably obtained, and accurate gradation can be obtained.

[0032]FIG. 9 is a diagram showing a pixel of a TFT-OELD related toanother embodiment of this invention. Here, only one pixel 11 isdepicted, but many pixels 11 actually exist in a plurality of lines anda plurality of rows.

[0033] An OELD (lower bit) 181 is formed of an OELD (lower bit.roundshape) 1812, and an OELD (upper bit) 182 is formed of an OELD (upperbit.first.round shape) 18212 and an OELD (upper bit.second.round shape)18222. The OELD (lower bit.round shape) 1812, the OELD (upperbit.first.round shape) 18212, and the OELD (upper bit.second.roundshape) 18222 are arranged at the same interval in horizontal andvertical directions within the pixel 11, and also with respect to theadjacent pixel 11. Because of this, the light emitting part of each OELDcan more reliably have the same area, and accurate gradation can beobtained.

[0034] Furthermore, as an EL element formed in each pixel, in firstembodiment (FIG. 7), a rectangular element is shown as an example, andin the second and third embodiments (FIGS. 8 and 9, respectively), around-shaped element is shown, but this invention is not limited tothese. Accurate gradation can also be obtained in a polygonal-orelliptic-shaped element. In particular, an elliptic element, as in thecase of a round shape, does not have a vertex such as is present in thecase of a rectangular shape, so there is no problem such that the vertexcannot be filled by the light emitting layer.

[0035] Each of FIGS. 7-9 provide lower and upper TFT's 171, 172,respectively, comprising on/off states contributing to the control ofgradation in the lower OELD (1811, FIG. 7; 1812, FIGS. 8 and 9) andupper OELD's (18211 and 18221, FIG. 7; 18212 and 18222, FIGS. 8 and 9)each respective TFT corresponds to.

[0036] As described above, according to this invention, by controllingthe area of an electro-luminescent element that emits light, accurategradation can be realized.

What is claimed is:
 1. A display device comprising a plurality ofpixels, each of the plurality of pixels including: a plurality of lightemitting elements having a substantially equivalent light emitting area,each of the plurality of light emitting elements being defined by abank; a plurality of driving transistors each having a resistance, theresistance of at least one of the plurality of driving transistorsdetermining a current amount flowing through one of the plurality oflight emitting elements, and the resistance of another of the pluralityof driving transistors determining a current amount flowing through twoof the plurality of light emitting elements; and a plurality of samplingtransistors, a gate of each of the plurality of sampling transistorsbeing connected to a scanning line.
 2. The display device according toclaim 1, wherein gates of at least two of the plurality of samplingtransistors are connected to one scanning line.
 3. The display deviceaccording to claim 1, wherein the plurality of light emitting elementsare organic electroluminescent elements.
 4. The display device accordingto claim 1, wherein the at least one of the plurality of drivingtransistors are connected to a signal line through a source or drain,and another of the plurality of driving transistors being connected toanother signal line through a corresponding source or drain.
 5. Thedisplay device according to claim 1, wherein the resistance of each ofthe plurality of driving transistors is determined by binary data. 6.The display device according to claim 1, wherein shapes of the lightemitting areas are of no vertex.
 7. The display device according toclaim 6, wherein the shapes of the light emitting areas are round.